The present invention relates to integrating into a thermal spray system a method for the continuous in-flight reduction of suboptimal feedstock deposition and the in-situ removal of debris, such as less adherent feedstock and surface preparation grit particulates, from the substrate and coating.
Referring to FIGS. 1 and 1A of the drawings, conventional thermal spraying is a coating method wherein a continuous flow of hot gas 1 generated in chamber 2 is forced to pass through an ejection nozzle 3, forming a divergent gas column 4 having an axis 5. The column 4 is coaxial with the nozzle 3 and extends from the nozzle exit to a substrate surface 6 where the gas column 4 is projected into a surface spot 7. Due to atmospheric air entrainment into the fringes of the gas column, the temperature within the gas column follows a Gaussian profile 9 (FIG. 1) where the temperature decreases with distance from axis 5. Air entrainment into the fringes of the gas column also causes the velocity of the gas to decrease with distance from axis 5, following a similar Gaussian profile 9. Peak temperatures in the thermal spray gas column (near axis 5) may reach values in excess of 10,000 degrees Celsius, while gas velocities can range from several hundred meters per second to supersonic speeds. There are two main methods to heat the gas:
1) A combustion chamber where a mixture of a combustive gas and oxygen or air is ignited and ejected at supersonic (and subsonic) speeds through a nozzle.
2) A plasmatron comprising an arc chamber where an electric arc is struck between a cathode and an anode while a mixture of gases is continuously fed through the chamber. The gas mixture is heated by the electric arc and is ejected through a nozzle as a high temperature, high velocity plasma stream. One preferred plasmatron capable of issuing a high enthalpy (HE) plasma stream is shown in U.S. Pat. No. 6,114,649 of Delcea.
Feedstock material is injected into the gas column via one or more injectors 10. It becomes entrained in the gas column which transfers heat and momentum to the feedstock material, causing it to impact with high velocity onto the substrate surface where it adheres to form a coating 11. Thermal spray coatings adhere to the substrate primarily by physical forces. Because of this fact, the substrate surface is typically pre-treated prior to the coating process by means of blasting with high velocity abrasive particulates to increase the surface roughness and provide anchoring points onto which the coating can adhere. Additionally, the particulates impinging on the substrate must be in the optimal temperature and velocity ranges in order to attain a molten status and speed sufficient to deform into a lamellar structure—commonly referred to as a splat—during impact, which increases the ability to bond physically to the underlying surface. In order to form a coating of optimal thickness, more than one layer of splats is usually necessary; in this case several overlapping passes are performed. A pass generally consists of the gas column axis moving relative to surface 6 as shown by arrow 8.
In conventional thermal spraying, feedstock materials are generally powders of different coating materials in sizes between several microns to tens of microns. The powder is injected into the hot gas column, typically by using a carrier gas flow. The hot gas stream transfers heat and momentum to the powder, causing it to melt and impact on the substrate surface to form a coating. Due to technological and economic constraints, thermal spray powders have a relatively wide spread of particle sizes, which is problematic because larger particles require more heat and momentum to form splats during impact than smaller particles.
In suspension thermal spraying (STS), the feedstock material consists of particulates suspended in a liquid medium. A flow of this suspension is used to inject the feedstock material into the hot gas column; thus, the liquid medium replaces the carrier gas used in conventional thermal spraying. Compared to conventional thermal spray powders, these particulates are significantly smaller, generally in the submicron to nanometer range. A range of solid particulate sizes is also present in the suspensions, but this range is generally smaller than that of conventional thermal spray powders. Upon injection into the hot gas stream column, the liquid solvent of the suspension is evaporated by the heat of the gas column. Afterwards, heat and momentum continue to be transferred to the particulates, causing them to melt and impact onto the substrate surface to form a coating.
The particle size spread found in conventional powders and in suspension feedstock is deleterious for the spray process. Ideally, all feedstock particulates should be entrained and travel in the hottest and fastest core region of the gas column along axis 5. However, the injection methods—either carrier gas or liquid medium—typically impart approximately the same velocity to all feedstock particles. Consequently, as shown in FIG. 1 of the drawings, only feedstock particulates 12 that are optimally-sized to the injection and gas column conditions stay near axis 5 of the gas column 4, which results in them impacting the substrate with the temperature and velocity necessary to obtain a high quality coating. The largest, heaviest particles 13 tend to penetrate farther through the gas column 4 and travel outside the core region in the cooler and slower region of the gas column 4 opposite the feedstock injector 10. In the cooler, slower region, particles 13 do not receive enough heat and momentum to form splats upon impacting on the substrate, so they do not adhere well to the substrate and form suboptimal deposits an annular region surrounding the central area of high quality coating. The smallest and lightest feedstock particles 14 likewise form suboptimal deposits in an annular region surrounding the central area of high quality coating, because these particles cannot penetrate into the core of the gas column and travel instead in the fringes where the temperature and velocity are suboptimal. Since a coating is typically produced by overlapping passes to produce multiple deposition layers, the suboptimal deposits can get entrapped in the coating, lowering the coating adhesion and integrity. As a result, the coating strength will be improved by reducing the formation or entrapment in the coating of suboptimal deposits. The formation of suboptimal deposits can be reduced by increasing the fraction of particles in the feedstock that are optimally-sized; however, narrowing the particle size range tends to increase significantly the overall cost of the coating process. Alternatively, the entrapment of unwanted suboptimal deposits can be reduced by cleaning these deposits off the surface between coating passes.
The techniques commonly used to clean unwanted material off a surface prior to applying a thermal spray coating involve directing a jet of pressurized gas onto the surface. Often times a compressed jet alone does not provide sufficient cleaning; so, solid particulates, such as dry ice or abrasive ceramic grit, are added to the jet to provide a more aggressive cleaning. In the case of abrasive grit blasting, coated areas adjacent to the region to be cleaned generally need to be masked or shielded from the grit to prevent damage to the coating. Additionally, the grit blasting process leaves dust particulates on the surface that can become entrapped in the coating and lower the coating adhesion and integrity. With these blasting techniques, equipment separate from what is needed for the thermal spray coating application is used, resulting in additional expenditures for equipment capital, maintenance costs, and coating production time if the thermal spray process is interrupted while the blasting equipment cleans the unwanted material.
One may argue that the feedstock injection could be stopped, and the hot gas column could be used to remove suboptimal deposits off the surface without the need for separate equipment. This approach is not feasible because the heat from the gas can partially or fully melt the suboptimal deposits, which can cause an increase in the adhesion of the suboptimal material after it cools. Furthermore, even though the adhesion of the suboptimal deposits may be increased by the hot gas column, the physical bonding and surface finish resulting from this melting and cooling process will not be comparable to that produced by the high velocity impact of molten particles.
U.S. Patent Application Publication No. 2009/0324971 A1 to De Vries et al. teaches an atomic layer deposition technique. No feedstock is injected into the plasma in order to deposit a coating having identical chemical properties with the feedstock. Rather, mixtures of reactive gases are fed into a reaction chamber and the plasma is introduced separately to enhance the reaction rate. Ions from the gases chemically bond to the substrate to form atomic layers. Water vapors are then injected cyclically along the substrate surface as a reactive agent which bonds to the surface in either an additive or substitutional manner to change the surface chemistry. Thus, De Vries teaches using more reactive species to break randomly the existing chemical bonds of undesirable atoms/molecules on the surface, resulting in the more reactive species replacing the undesirable atom/molecules and changing the chemistry of the surface. The technique in De Vries is not transferrable to a thermal spray process where the bonding occurs by physical instead of chemical forces. For example, it is the inventors' belief that even if for some unknown reason one might be motivated to inject water vapors along the substrate surface while thermal spraying a coating as taught in De Vries, it is not obvious to do so since it would likely not result in suboptimal feedstock particles being cooled sufficiently to prevent adherence, nor would the water vapor velocity be able to remove loosely adhered suboptimal deposits.
U.S. Patent Application Publication No. 2008/0072790 to Ma et al. teaches a thermal spray system using a combustion chamber and a nozzle to eject a plume towards a substrate. Feedstock material consisting of liquid media, which can include mixtures of organic/inorganic metal salts or suspensions of small-sized solid particles in water or a volatile solvent, is injected into the plume. The water and the solid particles are pre-mixed as a unitary feedstock and are supplied to the plume as a mixture from the same reservoir. The suspension liquid including water is employed by Ma as a carrier for the solid particles solely because of the difficulties to feed fine particles (under 10 micrometers in size) using gas as a carrier (para 0007). Ma does not teach the injection into the plume of a liquid such as water segregated from the solid particulates in the plume, and no provisions to achieve such segregation are disclosed within the description of the embodiments. Furthermore, Ma does not teach liquid injection to modify the deposition characteristics or structure of the coating being formed.
U.S. Patent Application Publication No. 2004/0203251 to Kawaguchi et al. teaches that semiconductor wafer manufacturing can produce residue that will release (“outgas”) gaseous reactants when exposed to atmospheric gases and water vapor. These reactants can cause contamination or corrosion issues to the part or processing equipment. (para 00026) To resolve this issue, Kawaguchi et al. describe using an apparatus generating a static, low temperature glow discharge plasma confined within a vacuum chamber to pre-heat the wafer containing the residue. (para 0031) Then, depending upon the residue chemistry, the wafer is exposed to an oxygen- or hydrogen-containing gas, either of which could be water vapor. (para 0029) This exposure releases the problematic reactants and converts them to into noncorrosive volatile species that are then removed from the vacuum chamber by pumping out the gases. (para 0030). The residue removal taught by Kawaguchi is in essence a reactive heat treatment performed statically under vacuum conditions and designed to convert the unwanted material into a gas. This process is specific to the chemistry and concerns of the semiconductor industry. Such a removal mechanism is not applicable to a thermal spray process performed in atmosphere with relatively nonreactive, non-chemically bonded debris that is best removed by mechanical dislocation, i.e. by the collision of particles with the debris.
U.S. Pat. No. 4,770,109 to Schlienger et al. teaches using a plasma torch, not to spray thermally-applied coating, but rather to heat and compact garbage onto a rotating disk located at the bottom of an incinerator chamber. After compaction and incineration, the treated garbage is emptied from the chamber, and the process is restarted. The torch is mounted through the upper lid of the incinerator with the plasma plume directed onto the rotating disk. The garbage to be treated can be in solid as well as liquid form. The solid and liquid garbage are not injected into the plasma plume; they are both fed through one pipe located away from the plasma plume (part 22 in the drawings and col 3 lines 6-7). Although Schlienger teaches feeding solid and liquid materials into a plasma produced by a plasma torch, the purpose of the process is to destroy the feedstock; therefore, Schlienger provides no provisions to be obviously usable in a thermal spray coating process which seeks to maximize the retention of the desired feedstock. Furthermore, Schlienger provides no provisions for a liquid to be injected directly into the plasma plume for the purpose of affecting the way feedstock particles are treated within the plume.
U.S. Patent Application Publication No. 2007/0084244 A1 to Rosenflanz et al. teaches the use of a plasma torch for treating feedstock materials for the purposes of producing amorphous or glass materials. Feedstocks of various ceramic particles are suspended in a carrier gas in order to be fed into the plasma plume. Once fed into the plasma plume of a given length, the feedstock particles are heated and melted into droplets. Rosenflanz makes no provision for also injecting a liquid into the plasma plume. Instead, Rosenflanz teaches spraying the plume and feedstock material into a liquid in order to cool the molten feedstock into particulates in the form of spheres or beads and separates this process from that of from producing a coating. (para 0104)
None of the above techniques or prior art provide a controlled in-situ removal of surface debris during a thermal spray coating process, while also reducing the deposition of suboptimal feedstock particulates in-flight. It should therefore be desirable to provide a thermal spray apparatus incorporating both of these means of avoiding the entrapment in the coating of particulates with suboptimal properties.